Hobble turning method and preferred applications for said method

ABSTRACT

A process for hobble lathing, and preferred process applications, for the cutting of workpieces with non-circular or discontinuous contours on programmable lathes. The use and combination of a special program e.g. of thread cycles and hobble values for the diameter and/or the longitudinal axis or the pitch, the angle of the spindle, in option of a reciprocal-step technique and interleaved machining sequences opens up virtually infinite possibilities. The preferred applications of the process allow special threads to be cut on screw-in bodies, e.g. screw-in artificial hip joint sockets and bone screws for example with neutral or virtually any angle of pitch or relief of the thread blade as well as e.g. internal and external contours on workpieces for circular wedge connections. A particularly beneficial hip joint socket is presented comprising so-called screw or threaded surfaces.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 10/935,558 filed on Sep. 7, 2004 now U.S. Pat. No. 7,513,913, whichapplication is a continuation of U.S. application Ser. No. 09/605,261filed Jun. 28, 2000 now abandoned, which is a continuation-in-part ofPCT application No. PCT/EP98/08473 filed Dec. 12, 1998, which claimspriority under 35 U.S.C. §119 of German Application No. DE 197 57 799.7filed on Dec. 29, 1997, and which is a continuation-in-part of PCTapplication No. PCT/EP00/05325 filed Jun. 8, 2000, which claims priorityunder 35 U.S.C. §119 of German Application No. DE 199 25 924.4 filed onJun. 8, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a special method for the turning of workpiecesand preferred applications of the method. The invention also concerns ascrew-in type artificial hip joint socket designed for cement-lessimplantation in the human medical domain.

2. Description of the Related Art

The principle of conventional lathing is a method which has been knownof for many years and is used for the cutting manufacture of workpieces,e.g. of wood, metal or plastic. In recent years, lathing technology hasundergone rapid advance due to the introduction and continuousdevelopment of numerical controls. Thus, today it is absolutely nolonger any problem to, for example, maintain a constant cutting ratealong a surface contour. With a suitable program it is now relativelysimple to produce even the most complex rotationally-symmetricgeometries in very short machining times. Furthermore, machines of thistype can be further upgraded by equipping them with tool drives becausethis allows even complex workpieces to be lathed and milled to form afinished product with a single clamping. Despite this, there are certainlimitations in connection with certain geometrical shapes or because ofthe time required. It is for example a fact that lathing in general hasconsiderably shorter machining times than does milling. In addition,turning yields better surface qualities. If as a result of the geometryof a workpiece it is only possible to employ milling techniques, it isunavoidable that either a considerably longer machining time is involvedor that a less uniform surface has to be accepted. However, thisnotwithstanding, even milling techniques are subject to certainlimitations as far as the geometry is concerned. Thus, for example anycorner of a milled contour in the radial plane of the milling axis cannever have a corner which is sharper than the radius of the milling toolused. And while it may be possible to produce sharper contours usingtechniques such as broaching, percussion and erosion, it is necessary totransfer the workpiece to a different machine for this end. In the caseof erosion the time requirement is also extremely long. While it is alsotrue that the cutting of non-circular contours has been possible for anumber of years now using profiling turning devices availablecommercially, these devices are expensive and therefore require acorresponding scale of capital investment. Furthermore such machines canonly be connected to the initially intended interface and are limited tothe specified contour with two-dimensional non-circular geometry.

In the past there have been attempts to enable lathes to machinenon-circular workpieces by fitting special mechanical modules. Onemachine of this type is proposed in the German publication DE 25 15 106.In addition to the very complex and very sensitive mechanicalconfiguration, this machine has extremely limited possibilities which inturn are themselves limited to the generation of two-dimensionalnon-circular geometries.

The geometrical possibilities for non-circular machining can be expandedwith respect to a tool which can be fitted to the lathe if for examplethe cutting drive can be controlled in a freely programmable fashion. Atool of this type is for example described in the German publication DE35 09 240 A1. In this case piezoelectric or magnetostrictive actuatorsare used in order to achieve a dynamic shift of cutting relative to theworkpiece using appropriate electronic controls. However, this techniqueonly allows extremely small adjustments to be achieved. While it wouldbe technically possible, for example, to use a magneto-dynamic system toachieve considerably larger control movements, these would as previouslybe limited to a single movement axis. In order to achieve specificthree-dimensional discontinuous machining it would be necessary to add asecond or possibly even a third orthogonally arranged movement unit tocreate a tool with complex directions of movement, whereby this would beof extremely complex design and demand highly sophisticated controlelectronics. To date a tool of this design is not yet available.

There are known other special turning lathes which have been developedfor non-circular machining, for example, of pistons for internalcombustion engines. Modern pistons have in fact a very slight oval crosssection, generally elliptical, in order to compensate for anisotropicexpansion during heating. Having said this, there is however only a veryslight deviation from the circular shape, whereby the contour also has avery flowing shape. There are no jumps or extreme discontinuitiespresent. This being the case, the constructional design of a machinewith this capability does not represent any major difficulty. Inprinciple it is sufficient to allow the tool to oscillate with a slightamplitude on the X-axis of the diameter whilst the carriage traversesthe workpiece in the Z-axis. In so doing the path of the tip of the toolwill follow a more or less sinusoidal curve such that extremeacceleration is not necessary. This latter would be very difficult toachieve despite the reduced mass of the system. It is pointed out thatsuch machines require a coupling of the workpiece rotation to themovement along the x-axis whereas the advance in the Z-axis can befreely chosen. In fact the generation of the non-circular contour isrestricted to the two-dimensional diameter plane and is only extended ina third dimension by way of the Z-axis. In reality the Z-axis is notactually involved in the generation of the non-circular contour. Thereis no technique for moving the carriage along the Z-axis in jumps orwith superimposed oscillation, for example.

A special machine of the type described above is described in the Germanpublication DE 40 31 079 A1, for example. In this case it is proposed tocontrol the drive required for the oscillating movement of the tool (forexample an electric linear motor or a hydraulic system) by means of anextra computer control in addition to the existing mechanical control,whereby this could be a personal computer, for example. However, amachine of this description would be limited in its possibilities to theintended and similar applications unless its basic kinematic method ismodified. Furthermore, a special machine of this description would berelatively expensive to acquire.

SUMMARY OF THE INVENTION

Therefore, the task at hand was to create a method for lathingworkpieces with irregular, discontinuous or abruptly changing contourswhich on the one hand makes use of the existing possibilities of amachine with compound slide and NC control, as well as in connectionwith additional equipment such as linear slides. On the other hand itshould overcome inertial problems and at the same time provide theoption of extending the degrees of freedom with respect to thediscontinuity of the intended contour by at least one additionaldimension. In so doing a further goal of the new method is to waive theneed for the previously necessary milling operations as far as possible.

The task referred to is solved by the invention using a turning methodwhich is described by the applicant as hobble turning (also referred toas jerk or limp turning). In this the workpiece is rotated in the chuckof the machine spindle at a preferably constant speed of rotation duringwhich the compound slide with the optionally fixed or controllablecutting tool is moved along the chosen axis, e.g. the pitch axis usinge.g. a thread program or a C-axis program synchronized to the spindleaxle to generate specific non-circular contours made up of combinationsof geometrical transitional elements using a program of jump functionsby linking command blocks with values for selected address parameterse.g. diameter (X), length (Z) and either angle (C) or pitch (F) wherebyfor at least one of these parameters in the program block chain asequence of hobble value groups is used with at least one numericalvalue in each value group. This method can be expanded by including theparameter height (Y) in suitably equipped machines.

In order to generate a specific tool track relative to the workpiece theover-shoot, the inertia, contouring error and the nominal all rigidityof the components concerned are all specifically exploited. However, itis in particular proposed to generate a tool track which only partiallycorresponds with the required discontinuous contour. It is then possibleto either remove the non-required, or unusable sections or thosesections non-compliant with the specifications in a follow-up machinecycle e.g. by milling, or to correct the contour to achieve the finalcontour using subsequent hobble machining. In the case of certain tasksit is especially advantageous to employ a jump system according to theinvention in which the discontinuity to be created using interleavedsequential sequences comprising geometrically opposed staggered lathingcycles.

When programming the control according to the invention to generate therequired tool movements in most processing tasks the increments formedbetween the numerical values for at least one address parameter in theprogram block chain represent a hobble sequence of value groups with atleast one numerical value in each value group, whereby for example thecorresponding numerical value within one value group is larger than thatwithin the other and/or the sign within one value group is positive andwithin the other value group is negative. In principle the programvalues in the program block chain for a certain address parameter form asequence of numerical values in which the commanded jump function isexpressed as hobble steps. In so doing the respective target coordinatescan be plotted as dots on a curve whereby they are connected by straightlines. It is part of the nature of the invention that in those sectionsof abrupt contour changes, in particular the tool track generated, willnot to be compliant with the straight lines but will approximate thestraight line in the form of rounded transitional functions.

The special significance of this method is its applicability in allthree dimensions, even without the inclusion of the height axis (Y).This machining freedom is due to the fact that the hobble steps can beprogrammed via X, Z, F and C either singly or in combination with oneanother as well as in connection with a tool with linear drive.

In its simplest form the method according to the invention requiresneither special equipment nor additional NC controls. It can be realizedbased solely on the use of the possibilities provided by the machinecontrol and appropriate software and is only limited by the dynamics ofthe overall system. This can comprise for example the known commandblocks G 01, G31, G 33, G 34, G 37 or G 131 etc., whereby for exampleaddress parameter diameter dimension (X), longitudinal dimension (Z),thread pitch (F), start-up length (B), overshoot length (P), start-upangle of the spindle (C), reference direction for F (H) and change inpitch (E) may be used or by inserting blocks with special software. Thepossibility is also not excluded that based on the method proposed herethe industry will in the future offer expanded programming possibilitiesas standard.

When driven special tools are not utilized, the dynamics described aboveof the overall system is made up of the mechanical and electronicdynamics of the machine. The mechanical dynamics is dependent upon themass of the compound seat and on the response speed of the drive, e.g.comprising threaded spindles, motors and gears. In contrast theelectronic dynamics is dependent upon the speed of the control methodand/or its links with the electrical motor drives. It is therefore thecase that lathes of the latest generation equipped with digital drivesand the fastest computers are suitable for extreme machining of ovalitywhereas the application of this method on older machines will havecorresponding restrictions. These restrictions can to a certain extentbe partially overcome by the use of reduced cutting speeds duringlathing because this results in lower spindle speeds and alsocorrespondingly reduces advance speeds.

A very simple application of this method comprises for example thelathing of eccentric journals. In this case for example an angularresolution of 180.degree. is realized with respect to the workpiece by,for example, linking command blocks, e.g. in this case G 33, by in eachcase programming the start-up co-ordinates in X and Z and a pitch in Fwhereby the increments lying between the programmed Z values of in eachcase 180.degree. for the angular step referred to must in principle havea value of half of the programmed pitch value. In contrast, the valuesfor X for each 180.degree. half step vary backwards and forwards betweena larger and a smaller programmed diameter value, whereby in theory theaverage value corresponds with the diameter of the journal and the halfdifference corresponds with the eccentricity of the journal. In order tosimplify the programming work, it is possible for example to enter therepeating jump(s) in the Z or in the diameter axis in some controls as avariable. Since in the example described the diameter change isgenerally larger than the intended advance, in this case the pitch, in anormal case the machine control will deduct the programmed pitch againstthe advance on the X-axis. Therefore it is necessary that for the pitch,the value F—i.e. the path programmed with respect to the diameter perrotation—must be entered as double the diameter difference, unless thereset is prevented by command blocks, e.g. with H. The programmingdescribed produces a theoretical track curve of the compound seat havingthe form of an extending zigzag line. In effect, however, because of thevarious ameliorating factors, e.g. the high mass of the compound seatand the insufficient rigidity of the control loop, the movement of thecompound seat during advance along the workpiece is actually acontinuously repeated quasi-sinusoidal curve such that despite the inprinciple primitive programming a remarkable roundness of the eccentricjournal is achieved. On the other hand this distortion means themeasurable dimensions of the workpiece do not correspond exactly withthe programmed values. It is therefore necessary to determine the actualprogrammed numerical values based on trial workpieces. Based on these itis, however, possible to reproduce the dimensions with high precision onthe machine concerned.

The procedure described above is applicable for the turned production ofelliptical bodies, in that the programmed zigzag curve is specified witha double resolution, i.e. with angular steps of 90.degree. In this casethe two alternating program diameters describe the theoretically maximumand minimum diameters of the ellipse. It is then necessary to programthe pitch which is usually calculated by the control along the X-axiswith a value of four times the diameter difference.

A similar procedure is then adopted if it is intended to produce apolygon (a so-called orbiform curve) whereby the resolution of theangular step must be 60.degree. Machining of this type is for exampleinteresting in the production of face-side cut grooves, as used todayfor example as the lubricating groove of starting discs or the cleaninggroove of disk brakes. Proper functioning in these cases does notrequire precision machined groove tracks, such that any track deviationscan be disregarded.

The examples described above are concerned with relatively harmoniousnon-circular items with a constant advance in the longitudinal axis withfixed and programmed pitch. It is easily possible to extend theprogramming described by the addition of auxiliary points in order toproduce perfect contours. The method according to the invention can beextended considerably further because it enables extreme jump machiningof the workpiece, also along the longitudinal axis.

To achieve this result it is proposed to use cutting techniques toproduce workpieces with even greater in particular spatialdiscontinuities and with angular contours or to achieve higher degreesof track precision by bringing in variable or stepped pitch values, forexample also in connection with a finer resolution of the contour. Inthe program the track to be followed by the compound seat in order toachieve a specific contour is described in the form of linked blocks,e.g. with G 33, with a different pitch specified in each program blockwhereby in extreme cases, e.g. a very small value for F followed in thenext program block by a very large value for F results in for example asequence of soft then abrupt movements of the compound seat. This methodallows the lathing of discontinuities of great diversity to be achievedfor example also the surface shell of curved bodies. It is possible in asimilar fashion to use this method to achieve discontinuous contouroutlines as described by using co-ordinate chains programmed in theprogram block made up of only respective X and Z values or also inconnection with F values. Thus for example the advance in one or bothaxes can be programmed as reciprocal-steps whereby after a certainadvance movement there follows an in each case abrupt (shorter) returnjump which is in turn followed by for example a larger advance distance.In this sense such a method can for example be understood as being thealternating cutting of linked right and left hand threads with undercertain circumstances asymmetrical thread pitches.

The method according to the invention also allows the cutting ofdiscontinuous contour elements protruding from an angled or curvedsurface shell whereby the side of the tool predominantly works the flankof the discontinuous contour element and the tip of the toolpredominantly cuts the surface shell. In this case suitable programmingof start and finish points and pitch allows the tip of the tool to becontrolled along a track which for the most part runs tangentially tothe surface shell and the side of the tool generates the flank of thediscontinuous element controlled by a programmed change of the travelspeed and/or travel direction.

In the programming described particular care must be taken to ensurethat the reference direction for F, which is generally described withaddress parameter H, is correctly used. As is known, H describes whichaxis is used to calculate the advance which corresponds with the threadpitch programmed under F. Without other specifications or where H=0, theadvance refers to the Z-axis, i.e. in principle to longitudinally,conical and similarly linked threads up to maximum 450 to the Z-axis. IfH=1 then the advance calculation now refers to the X-axis, i.e. tobasically planar, conical and correspondingly linked threads of maximum45.degree. degrees to the X-axis. In this case H=3 refers to movement onthe thread track. In the case of linked threads on curved surfaces itcan easily occur that the limit value of 45.degree. is exceeded and themachine control then automatically springs over to the other axiscalculation. This must be either determined for example by conversionand be deliberately falsified in the program or this reset must beprevented by appropriate software in the event that the control systemhas such a command block available, e.g. with I for a face pitch and Kfor a I longitudinal pitch.

To complement this the programming of the target coordinates X and Z inconnection with the pitch F using a command block for threads (e.g. G33) has the problem that the actual pitch of zero will not be acceptedby the control. One possibility of overcoming this obstacle is to setthese parameters to the minimum programmable increment (e.g. 0.001 mm).

In the case of the invention, however, there is an even more elegantmethod to eliminate this problem whereby this also simultaneously avoidthe reset at 45.degree. as well as reduces of the programming work. Inthis method the hobble program e.g. in command block G 01, is specifiedin the form of coordinate chains of X and Z, and the spindle angle C.This waives the need for calculation of the respective pitch becausethis is derived from the difference between the in each case selectedreference parameters (Z or X) in relation to the spindle angle C. In thecase where the angular steps between sequential spindle angles in thecommand blocks are the same or all repeat themselves within a specificregularity, e.g. as a hobble rhythm, then the value for C can beprogrammed as a variable. In this case the parameter is either raised orlowered in value, after completing of the respective program block, byan amount equal to the respective angular step value which can also beprogrammed as a variable or as a fixed value. In the event that changesare required to the, under certain circumstances extremely long,programs it is then generally possible to modify only a smaller numberof fixed values or variables.

The method described above for spindle programming is however onlysuitable for certain machines and NC controls which are compliant withstate-of-the-art developments. In these machines the spindle isintegrated in the drive motor whereby the entire unit can be addressedeither as the turning axis or as the C axis. With correspondingly fastNC controls there is a certain degree of a equivalence with respect toprogramming between the speed of turning of the spindle, which is forexample expressed in that the C axis can be used even at very high rpm(under certain circumstances several thousand rpm). This means that theprogramming of the C axis allows cutting speeds to be achieved which arecomparable with those of standard lathing operations.

The overall method according to the invention is further extended by theproposal to overcome application limitations due to the restrictions ofmachine dynamics or of the linear driven tool in that for extrememachining geometries an interleaving of the processing sequences isemployed. This refers to a kind of jump method in which for example afirst machining cycle produces a first contour element but which at thesame time also skips a second in order then to follow a third contourelement when its tracking has steadied, and so on. The contour elementsmissed out of the first machining cycle are then cut in a secondmachining cycle, whereby the contour elements of the first machiningcycle are now skipped. This method takes into account that the overrunof the overall system as a result of an abrupt movement programmed atmaximum traversing speed means that the overall system is not able totrack a contour element which follows at a close distance and will notbe traversed in the desired manner. Although in order to execute themethod two or more machining cycles may be required, which takes longer,this nevertheless represents a drastically shorter time than thatrequired by milling techniques.

Together with the invention preferred applications of the method arealso proposed. These applications also serve to provide a more detailedexplanation of the method based on a number of application examples.

The proposed application concerns the production of threads for diverse,in particular self-tapping screw-in bodies into yielding materialswhereby such bodies are e.g. wood, plastic and bone screws includinge.g. implants such as lag screws, vertebral fusion bodies, screws forfixateur externe, screw in posts for dental implants and artificial hipjoint sockets.

A further application is the inexpensive manufacture of so-calledcircular wedge profiles on the internal or external coupling faces ofcoupling elements in mechanical engineering.

One of the above proposed applications refers preferentially toself-tapping artificial hip joint sockets for cement-free implantationinto humans. These kinds of screw-in type artificial hip joint socketsare available commercially in various designs. In order to ensurereliable and permanent integration and also simplified handling duringimplantation surgery the design of the thread is of primary importance.It is known in the interim that a large contact area of the implant tothe bearing surface without stress peaks and a threaded profile inclinedtowards the pole of the socket help create the best preconditions toavoid loosening. On the other hand, such a screw-in type artificial hipjoint socket must provide good tactiliance, which is a term whichdescribes the “feel” of the surgeon for the seating of the socket bodyon the prepared bone surface in the acetabulum during the screwing in ofthe screw-in type artificial hip joint socket. In existing screw-in typeartificial hip joint socket types there is a need for improvementbecause they either leave undesirable free spaces to the bone interfaceafter implantation or can only be screwed in with excessive force ortheir tactiliance is insufficient, i.e., the surgeon does not “feel”when the socket has seated in the bone.

One group of screw-in type artificial hip joint sockets is configuredwith a so-called flat screw in which the lateral surfaces of the threadrib are parallel to one another. It is standard procedure to interruptthe thread web by machining tapping groove(s) at certain intervals inorder to form cutting edge(s). In this type of thread the cutting forceduring self-tapping must be applied totally via the radial head surfaceof the thread rib which is inclined outwards or by any cutting edge(s)which are in situ there. When these other measures are undertaken,however, the head surface of individual thread teeth describes a spiralcurve in the axial view of the pole-side of the screw-in type artificialhip joint socket, the exact track of which is dependent on the form ofthe shell body of the screw-in type artificial hip joint socket and thepitch of the thread. As a result the radial curve spacing from the polarcenter increases with progressive turns. The end of any one thread toothis therefore at a greater radial distance outwards than at its start.This means that during screw-in of such a screw-in type artificial hipjoint socket a pinching effect is created which can only be amelioratedby the filing forces of the roughened surface of the implant on the bonematerial. This means that implants of this design have unnecessarilyhigh screw-in forces.

On the other hand, screw-in type artificial hip joint sockets areavailable with a flat thread, the thread teeth of which have a reliefangle created by over-milling in groups. However, as a result of themachining technique chosen, a number of straight head-side surface(s)are created which run back as chords which are offset to the respectivewheel circle formed by the respective cutting edge(s). This means that,although screw-in type artificial hip joint sockets with this kind ofthread are relatively easy to screw in, they only have a reduced contactarea to transfer forces because of the shortened thread tooth height. Aspecial disadvantage is the formation of gaps in the area of the threadtooth head (individual thread teeth), between the implant and the bone,as well as the leverage forces acting on the bone substrate because ofthe excessively deep cut of the tooth flutes. This is the reason whyscrew-in type artificial hip joint sockets of this type are also deemedmedically deficient.

Screw-in type artificial hip joint sockets of the type described abovewith a flat thread have only been able to claim a small fraction of themarket to date. At the present time, screw-in type artificial hip jointsockets with so-called pointed threads are more wide-spread. However,these products are burdened in principle by the previously describedcomplex problem with respect to unacceptable screw-in characteristicsand the formation of a gap in the contact zone. The various attemptsmade to reduce the screw-in forces have actually, amongst others,resulted in an excessive widening of the milled tapping groove(s) to thedetriment of the thread teeth. This means that valuable contact area islost in conjunction with the formation of extended cavities and alsoreduced osary areas to absorb the forces.

In U.S. Pat. No. 4,997,447 a screw-in type artificial hip joint socketwith round thread flutes is proposed in which the head surfaces ofindividual thread teeth run in a curve, whereby a relief angle isrealized which reduces as the radius of this curve, seen from the socketpole, reduces with increasing distance from the cutting edge(s). In thisscrew-in type artificial hip joint socket, the degree of gap formationrelative to the straight head surfaces is noticeably reduced without anyloss of good screwing properties. However, this configuration does alsonot result in a full bone contact of individual thread teeth in therespective rear zones. Furthermore the manufacture of this product isextremely time-intensive, because the proposed design requires thecomplete traversal of the tooth head length with a milling machine.

Up to now, no products are available commercially in which screw-in typeartificial hip joint sockets with pointed threads have individual threadsegments with a relief angle. This is thought to be in connection withthe fact that the implementation of such a design is extremely difficultand the initial choice of adopting milling techniques for productionwould require not only extremely complex programming but also veryextensive machining times. These problems are due to the problem that inthe case of pointed threads and depending upon the pattern of thetapping groove(s) at least one of the lateral surfaces of the threadtooth must be used to form a cutting edge(s). If, however, a neutral orrelief angle is to be formed behind the cutting edge(s) then thecorresponding lateral surface of the respective thread tooth must bebackmilled to the subsequent tapping groove(s) at a congruent lateralangle. This creates the problem that the milling machine cannot machinecurved surface shells while simultaneously following the contour of thebase of the thread flute. One has then the choice of either accepting anincreasing groove-like depression along the flank of the tooth or acorrespondingly large stepped residual relict. In cases where thisrelict is unacceptable, it would then have to be removed subsequentlyusing at least one additional milling run.

With the method according to the invention it is, however, possible tocut such threads for hip joint sockets with great perfection in a shorttime using lathing techniques. In so doing it is irrelevant whether thediscontinuity machining to create a certain pattern e.g. a relief orneutral angle of individual thread teeth is to take place on its pole,its equator or its head side surface or on several of the surfaces.Because of the free programmability of the machining track it is notonly possible to master any desired profile of the thread tooth but eventhe angular pattern of the generated thread rib sections are virtuallyfreely selectable in three-dimensional space. At the same time theentire thread can be perfectly adapted to the outer shell of the socketbody. Thus the invention can be applied to all known shell forms, e.g.spherical, aspherical, parapherical, conical-spherical, conical,cylindrical, parabolic, toroidal, etc.

The method according to the invention can be simply combined with otherwell-known methods for the production of threads for hip joint sockets,e.g. with the method as described in European patent EP 0 480 551 (CA2,052,978) or with the method proposed in German publication DE 44 00001 (U.S. Pat. No. 5,997,578) for the production of a thread withmodifiable thread profile. A particularly beneficial combination appearsto be a thread tooth profile inclined towards the socket pole and athread pitch which changes smoothly according to international patentapplication WO 97/39702 (U.S. Pat. No. 6,146,425).

It is suggested in this regard in the invention that for artificial hipjoint sockets with a tooth profile which tapers towards the head of thethread tooth, that the thread teeth formed between the tapping groove(s)are produced as so-called screw surfaces (sometimes referred to asscrewed surfaces) and to selectively swivel them in their respectivedirection of extension depending upon the windup of the tappinggroove(s). In this case screw surfaces are understood to mean thosesurfaces which are created by the rotation of a certain tooth profilewith constant radial distance from the axis of the socket and with apitch around this axis. In the case of for example trapezoidal toothprofiles this would mean three screw surfaces are formed, one on thehead side and two on the lateral sides. In so doing, these screwsurfaces may become shortened in their base area along their extensionas the tooth profile sinks into the surface shell for certain shellgeometries of the screw socket. The surfaces which follow the cutter atthe start of the respective thread tooth will then have a neutral angle,i.e. neither a pinch nor a relief angle. This then avoids theundesirable pinching effects while at the same time ensuring bonecontact on all sides of the thread tooth. In order to enable the cuttingedge(s) to have the optimum effect at the start of each respectivethread tooth, it must protrude in comparison to the leading thread tooth(preceding tooth in direction of screw rotation). In the first step thisis achieved in that a larger radius is selected for the screw surfacesof a following thread tooth than for the screw surfaces of the leadingthread tooth. Preferably, individual thread teeth are swung relative toone another in their extension as a function of the windup of thetapping groove(s), whereby the preferred direction of swing is one whichapproaches the windup angle in order to realize an overstand of thelateral cutting edge(s) with a positive cutting angle.

Another practical implementation of the invention in the production ofthese types of threads is to generate overshooting transition functionsof the cutting track in specific positions on the thread length byprogramming hobble-jump(s) and to synchronize these with some form ofinterruption, e.g. in the form of tapping groove(s) such that duringmilling of the discontinuity the interfering or unusable parts of thecontour generated are removed and that the cutting edge(s) following thediscontinuity in the screw-in direction protrudes compared with thepreceding tooth profile. The remaining part of the tooth blade thendrops back compared with the cutting edge(s) such that behind thecutting edge(s) an area corresponding with a clearance angle (reliefangle) is formed.

A further application of the invention concerns so-called circularwedges (or 3K couplings) in general mechanical engineering. Thesecomprise a friction contact expanding coupling, for example betweenshaft and hub, which is a self-locking but releasable connection.

In the case of a circular wedge coupling, and in contrast to cylindricalcross-pressure locks, the joint connecting areas of the shaft and thehub are not round but have so-called wedge surfaces on thecircumference. Generally there are three wedge surfaces. They compriseidentical and reciprocally opposed turned sections of spirals, e.g.logarithmic spirals. When clamping by turning through a certainrelatively small angular amount (e.g. 15.degree.) the necessaryhomogenous contact surface is achieved, and hence the maximum possiblefrictional connection between the shaft and the hub. Circular wedgecoupling also provides for an excellent transfer of the respectiveforces and boast an advantageous rigidity of configuration. A couplingwith three circular wedges on the circumference is self-centering. Ifthe radial pitch of the wedges surfaces is selected between 1:50 and1:200, circular wedge couplings are generally also self-locking.

If produced in sufficiently large numbers, and if the technicalrequirements are not too demanding, circular wedge profiles can beproduced without cutting and hence relatively inexpensively. On theother hand, relatively smaller numbers, and to fulfill higher qualitydemands, has to date required either milling or grinding techniques withcorrespondingly high costs. The diameter of the milling tool or of thegrinding disk results in the creation of transitions to the individualcircular wedge areas which are unusable. In conjunction with the angleof twist required relative to the joints this means the coupling canonly transfer a fraction of the potential forces.

Using the method according to the invention circular wedge couplings ofthis type can be manufactured using interleaved machining sequences withgreater precision and at lower costs, even in small production numbers.The option is also created of machining couplings of this kind with aconical design if required.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail with respect to the preferredapplications based on twenty four schematic drawings. These are asfollows:

FIG. 1 Hemispherical screw-in type artificial hip joint socket with flatthread jamming on the head side according to state-of-art technology

FIG. 2 Hemispherical screw-in type artificial hip joint socket with aflat thread having a clearance angle according to state-of-arttechnology

FIG. 3 Hemispherical screw-in type artificial hip joint socket machinedaccording to the invention with a flat thread made up of thread teethwith head-side screw surfaces

FIG. 4 Hemispherical screw-in type artificial hip joint socket accordingto the invention with a pointed thread made up of thread teeth withscrew surfaces on all sides

FIG. 5 Two thread teeth of the screw-in type artificial hip joint socketaccording to FIG. 1

FIG. 6 Two thread teeth of the screw-in type artificial hip joint socketaccording to FIG. 2

FIG. 7 Two thread teeth with clearance angles and arc-shaped headsurfaces

FIG. 8 Two thread teeth of the screw-in type artificial hip joint socketaccording to FIG. 3

FIG. 9 Two thread teeth of the screw-in type artificial hip joint socketaccording to FIG. 4

FIG. 10 Three thread teeth of the screw-in type artificial hip jointsocket according to FIG. 3 and high-dynamic tool track

FIG. 11 Three thread teeth of the screw-in type artificial hip jointsocket according to FIG. 3 with a tool track of average dynamics usingthe hobble method

FIG. 12 Three thread teeth of the screw-in type artificial hip jointsocket according to FIG. 3 and over-swinging tool track with the hobblemethod

FIG. 13 Theoretical tool track generated with jump commands

FIG. 14 Workpiece contour generated from transition functions

FIG. 15 Final workpiece geometry after further processing

FIG. 16 Sleeve for a circular wedge coupling

FIG. 17 Journal for a circular wedge coupling

FIG. 18 Half-side sectional view of the screw-in type artificial hipjoint socket according to FIG. 3

FIG. 19 Half-side sectional view of the screw-in type artificial hipjoint socket according to FIG. 4

FIG. 20 Tool track program for a single jump function

FIG. 21 Tool track program for a double jump function

FIG. 22 Program for parallel tool tracks

FIG. 23 Program for interlaced tool tracks with jump functions

FIG. 24 Developed thread rip procession with thread blades havingclearance angles

DETAILED DESCRIPTION OF THE INVENTION

The drawing in FIG. 1 presents the pole-side view of a hemisphericalscrew-in type artificial hip joint socket 1 with a flat thread accordingto state-of-the-art based on an example with a 1.3 magnification. In theexample the nominal diameter is 54 mm, the average tooth height is 2.6mm, the pitch is 5 mm and the bottom hole diameter is 22 mm. These basicdimensions were selected for technical drawing reasons and are alsoretained in drawing FIGS. 2 through 4 to allow better comparability.Similarly, the windup angle of the tapping groove(s) has been set at0.degree. throughout in order to reduce the drawing work. It is knownthat a woundup tapping groove(s) offers advantages with respect to amore favorable cutting angle and a more evenly distributed transfer offorces.

A dome shaped thread-free area 6 on the shell body continues from thebottom hole 9 of the screw-in type artificial hip joint socket 1. In thedrawing the diameter of the shell body is represented by the equatorialmargin area 10. The thread starts on the pole side at first thread tooth7 and reaches its full height before thread tooth 2. Two of the threadteeth 2, 3 are marked with identifying numbers and are further detailedin detail drawing FIG. 5. Both the head-side surface(s) 4 and the edges5 formed at the base of the tooth at the shell body of individual threadteeth—with the exception of the starting and end zone of the threadlength—appear to be on a spiral-shaped curve in the two-dimensionaldrawing. The overall thread length has approximately 4 circuits. Thethread groove bottom 8 running between the thread teeth forms thehemispherical shell of the shell body. In order to create tappinggroove(s) 11 or cutting edge(s), the circumferential thread rib isslotted twelve times without wind-up. In so doing the slotting dips atan angle of around 10.degree. in order to form in each case a positivecutting angle at the individual thread teeth.

The example shown in FIG. 2 of a screw-in type artificial hip jointsocket with a flat thread according to state-of-the-art is produced fromscrew-in type artificial hip joint socket 1 by after-milling. In thediagram the bottom hole 20, the dome area 17, the thread groove bottom19, the nominal diameter 21, the slotting 22, the edges 16 between thethread teeth and the shell body all correspond completely with FIG. 1.In order to maintain a constant average threaded tooth height the threadteeth were individually milled because of the hemispherical shell body.In so doing the pole-side start of the thread moved to thread tooth 18.The straight outer surface(s) 15 of individual thread teeth now run aschords to the wheel circle of the leading head-side cutting edge(s) inthe screw-in direction and in synchronization with the thread slotting.such that relief angles are formed with respect to the respective wheelcircle. The effect of the cutting edge(s), of reducing the screw-inforces, is achieved by the circumstance that the radial distance of thecutting edge(s) from the socket axis is always larger than thecorresponding radial distance of the leading edge of the blade. Two ofthe thread teeth marked 13 and 14 are detailed below in FIG. 6.

The example illustrated in FIG. 3 is a screw-in type artificial hipjoint socket 23 machined according to the invention method andcorresponds in its hemispherical shell, its basic dimensions, bottomhole 31, dome area 28, the edge 27 between the thread teeth and theshell, the base of the thread 30, the diameter 32 and the threadslotting 33 with the example in FIG. 1. The thread length of the flatthread starts with a reduced tooth height in the first thread tooth 29which then increases in jump(s) in the next sequence of four threadteeth until the thread rib reaches its full height in thread tooth 24.The parallel flank(s) of each individual thread tooth border in eachcase on the outerlying section of a cylindrical surface 26 which iscoaxial to the screw-in type artificial hip joint socket axis, wherebythe basic cylinder diameter increases in steps from thread tooth tothread tooth. This design principle can also be achieved with arespective section from a correspondingly coaxial screw surface. Thisdesign as described forms neither a pinch nor a relief angle at thethread teeth. Indeed a relief angle is absolutely not necessary becausethe surface roughness (e.g. caused by sand blasting of the screw-in typeartificial hip joint socket surface) creates filing forces which,assuming a neutral relative movement, prevent any sticking during thescrew-in method. This means the undesirable formation of a gap betweenthe implant and the bone layer is avoided. Despite this, the frontouterlying cutting edge(s) of the thread tooth is effective because ithas a larger radial distance from the socket axis than the leadingcutting edge(s). This results in slightly lower screw-in forces withaverage tactiliance and improved primary and secondary fixation of theimplant.

The example of a hemispherical screw-in type artificial hip joint socket34 machined according to the method according to the invention isillustrated in FIG. 4. Here again the various individual details, i.e.the bottom hole 42, the dome area 39, the base of the thread 41, thediameter 43 and the thread slotting 44 are the same and unchanged fromthe previously described examples. In contrast to these, however, thethread described is a pointed thread comprising in principle atriangular thread tooth profile. This fact is not apparent from thetwo-dimensional drawing. In a similar fashion to the above, the threadlength commences with an initial small thread tooth 40 and the toothheight increases step-wise over several stages and reaches its final(average) tooth height at thread tooth 35. The edge 37 formed at thetooth head (individual thread tooth), which in the case of a pointedtriangular cross section of the threaded tooth is practically only aline, comprises for each individual thread tooth a screw line withconstant distance from the axis of the screw-in type artificial hipjoint socket which is shown in the diagram only as an curve with a fixedradius to the socket center. In the case of the pointed thread chosen,the lack of windup of the tapping groove(s) 44 means a cutting edge(s)is formed at both thread tooth flank(s). The cutting edge(s) would shiftto one of the threaded tooth flank(s) if the tapping groove(s) had therebeen a corresponding wind-up (tapping groove slant). The surfaces onboth sides of any individual thread tooth of the example shown comprisescrew surfaces whereby the pitch of the pole-side surface correspondswith the pitch of the equator-side surface even if the opticalimpression seems to indicate otherwise because of the socket diameterwhich increases towards the equator. Because of this, the edge 38 formedat the base of the tooth between the thread tooth and the shell of thescrew-in type artificial hip joint socket appears to run backwards intothe shell. After adopting a larger radial distance from the socket axisfor the screw surfaces of the next subsequent thread tooth duringscrewing in, the cutting edge(s) on both sides can be either lateral tothe thread profile of the leading thread tooth or protrude radiallyoutwards and will, as such, cut easily during screwing-in. In this caseagain, the neutral angle created by the extension of the thread toothmeans that the creation of gaps in the contact area to the bone isavoided.

The statements made in the preceding regarding state-of-the-art and theexamples of the method according to the invention are explained ingreater detail in the magnified details presented in the followingfigures because certain details are only difficult to recognize in theoverall diagrams.

In FIG. 5, two thread teeth 2, 3 are enlarged from FIG. 1. Of these,thread tooth 2 has a cutting edge 45 located on the front of itshead-side surface 46 and thread tooth 3 has a identical cutting edge 47on the corresponding surface 48. The wheel circle 49 which has a fixedradius around the central axis of the socket and which is described bycutting edge(s) 45 during screwing in of the screw-in type artificialhip joint socket is marked in as a dot—dash line. It is easy to see thatpart of the respective thread teeth extends beyond the wheel circle,which in general will lead to blocking effects.

FIG. 6 shows details of thread teeth 13, 14 according to the exampleillustrated in FIG. 2 and will not result in blocking effects becausethe surfaces 51 and 53 on the head side following cutting edges 50 and52 are milled with a relief angle. In so doing the dash-dotted wheelcircle 54 of cutting edge 50 does not touch the head-side surface of thethread tooth at any point. It is, however, true that each of these casescreates undesirable free play. This free play is larger, the smaller thenumber of tapping groove(s). This means that in particular screw-in typeartificial hip joint sockets with for example only six tapping grooveswill be extremely disadvantaged. The design shown is frequently used forconical screw-in type artificial hip joint sockets because then thethread teeth can be very rationally milled in so-called packages.Medically speaking, however, this argument bears no weight and should berejected.

The problem described above can be ameliorated to a certain extent byadopting a design of the thread teeth 60, 61 according to FIG. 7. Hereagain the head-side surface(s) 56, 58 of the thread teeth (i.e., apex,crown, ridge) have a relief angle with respect to the wheel circle 59behind the leading cutting edge(s) 55 and 57. This effectively preventsjamming during screwing in. However, because of the curved shape ofsurfaces 56, 58, the gap-forming free play is relatively small and istherefore more acceptable. On the other hand, however, this arch shapeis concomitant with a much greater milling complexity and effort becauseindividual thread teeth have in principle to be tangentially traversedindividually during manufacture. In the method according to theinvention the geometrical configuration illustrated of individual threadteeth can be produced much more rationally in only a single clamping ona CNC lathe.

In comparison, the configuration of the respective outer surfaces ofindividual thread teeth—as so-called screw surfaces—using the methodaccording to the invention, and as described previously in FIG. 3, isshown in FIG. 8 in the form enlarged depictions of two thread teeth 24,25. The head surfaces 63, 65 of the thread teeth extending from cuttingedges 62 and 64 respectively have a fixed radius which is defined ineach case as the distance of the cutting edges from the screw-in socketaxis 67. Therefore the wheel circle described by cutting edge 62 anddepicted in the drawing as a dash-pointed line with fixed radius 66 iscoincident with the head surface 63. Since the corresponding radius ofthread tooth 25 is larger, its cutting edge 64 extends or projectsbeyond the leading cutting edge 62 of thread tooth 24 during screwingin. This means that the respective cutting edge(s) and the subsequentfront area, set at a positive cutting angle, both penetrate/cut into thebone material and can transport the cuttings away in the tappinggroove(s) with a relatively light cutting force.

The situation in FIG. 9, showing an enlargement of a section of FIG. 4differs from that described in FIG. 8 in that the thread does not have aflat thread in its tooth profile but a pointed thread. Here again,however, the outer surfaces of individual thread teeth 35, 36 are eachdesigned as screw surfaces. Because of the inclined lateral angle andthe pitch or the angle of the thread teeth, and the hemispherical shellcontour, the edge formed at the base of the tooth to the shell jacketappears to run into the edge at its rearward end 73, 74. In fact,however, when the screw-in type artificial hip joint socket is rotatedthere is no radial shift of the projected tooth cross section becausethe respective outer edges 69, 71 are unchanged in their radius to thescrew-in type artificial hip joint socket axis. By bringing in atriangular tooth cross section for the example shown, there is a shiftof the respective cutting edge of at least one lateral surface of therespective thread teeth, and in the case of tapping grooves withoutwind-up, on both lateral surfaces. The drawing shows only the pole-sidecutting edge 68, 70. The respective rearward cutting edge is hidden. Thewheel circle of the head-side threaded tooth edge 69 is shown with fixedradius 72 around the screw-in type artificial hip joint socket axis 75.The extremely reduced screw-in forces of this design are the result ofthe mutual radial offset of individual thread teeth as a result of whichthe individual cutting edges stand out both laterally and outwardlycompared with the respective leading cutting edges.

In order to understand the procedure to implement the method for theproposed preferred application for the production of a screw-in typeartificial hip joint socket thread the features presented in FIGS. 3 and8 are again referred to in FIGS. 10 through 12. In each of the Figs. thethree thread teeth 24, 25, 76 of the flat thread are depicted as iscutting edge 62 on the head-side surface 63 with its dash-dot wheelcircle 77, with the radius 66 around the screw-in type artificial hipjoint socket axis. The scale of the figures is slightly reduced comparedwith the preceding figures.

FIG. 10 illustrates the track 78 of a machine tool (e.g. indexingcutter) which is equidistant to the head-side surface of the edge formedby the tooth head, whereby the track is achievable in the configurationshown using a program according to the invention comprising a smallnumber of target points (locations) with an extremely dynamic lathe or acorrespondingly dynamically driven tool. The distance of the track fromthe contour to be cut was selected in order to make the course of thetrack visible over its entire length. Track 78 contains twodiscontinuities 79 and 80 which are deliberately placed in thosepositions by the programming in order to allow subsequent machining ofthe slotting of the thread using milling techniques. Although thediscontinuities 79, 80 of track 78 are transitory in function, it hasthe effect of creating a radial jump function between sequential threadteeth. This radial jump function exists in every case with respect tothe proposed programming whereby at least two sequential followingco-ordinates of the same diameter have to be entered with a traverse inZ adapted to the machining task and a suitable pitch or suitable spindleangle and followed by a diameter jump at maximum advance speed (e.g. 100mm/rev). In order to achieve an acceptable machining result it isnecessary that the transition area on the workpiece is not wider thanthe intended width of the tapping groove(s).

The creation of the cutting track as shown in FIG. 10 is not evenpossible using a linear drive tool because the overall dynamics of thesystem are insufficient in order to move any compound seat with thenecessary precision within the required path on a different lathingdiameter. With the invention the proposal in this case is a jump methodwith which this problem can be overcome in principle. The correspondingtheoretical background is clarified in FIG. 11. The machining procedurefor track curve 81 suggests only machining for example the 1st, 3rd,5th, 7th etc. thread teeth in a first machining cycle and skipping the2nd, 4th, 6th etc. In this case the transitional function of track 81arising from the programming of the jump function and in connection withthe machine damping need only be sufficient such that after location 82the reaction is, for the tool to be lifted over the next followingcutting edge, merely enough not to round it off or damage it. There isroom up to location 83 to return the tool to the desired track, and thisis not limited by the width of the tapping groove(s). It is thenpossible without difficulty in a second machining cycle to complete thecontour elements skipped and to similarly skip those machinedpreviously.

In the case of older lathes with corresponding inertia in controlcircuits it must be taken into account that an over-response will resultin a distortion of the track curve. This effect is shown clearly intrack 84 in FIG. 12. Following the abrupt reaction of the tool movementto the programmed task at location 85 there is an over-oscillation ofthe track which reaches its maximum at location 86. This is thenfollowed by a soft build down transition until the track is again on theprogrammed course at approximately location 87. In this example thedescribed effect would still be just about controllable using thesuggested jump method in two machining cycles. If necessary the jumpmethod could, however, be extended to comprise of three or more cycles.

The variations as above describe a method which is equally applicable toinclined tooth head surfaces as well as to the lateral surfaces ofthread teeth, for example as per FIG. 9. In this the described jumpfunction is shifted either completely or partially from the X-axis tothe Z-axis. In these cases the hobble tracks described by the tool havenot been illustrated in the drawing, but do correspond in principle tothose jump methods shown for the machining of tooth heads (individualthread teeth)s.

As described previously the invention also opens up the possibility ofdirectly exploiting the overshoot behavior of the machine for thecreation of relief angles on thread teeth. The exact procedure isdescribed in more detail in FIGS. 13 through 15. FIGS. 13 through 15show three curves on an enlarged scale based on the example of staggeredtooth flank which have been reduced to the interesting movement sectionof the tool track for transparency by leaving out the spatialcomponents. In practice this movement could be on one or more levels.

FIG. 13 shows the tool track 88 commanded in the program using a singlejump command. Coordinate locations 89, 90, 91 and 92 are specified usingcorresponding values for X and Z. Of these only the modification of Z isa shown on the drawing sheet as vertical components, whereas therespective value of X is not apparent in the drawing. The horizontalspacing between the coordinate locations is proportional to therespective spindle angle, which can be programmed either directly viaparameter spindle angle C or indirectly via the pitch (F). In so doingit should be noted that if parameter F is also used the maximumpermitted value of the pertinent NC control must not be exceeded,whereas in the case of the spindle angle programming of the angularjump, 0.degree. can be set without problems. In principle a number ofjump commands can also be linked with one another.

FIG. 14 shows that the configuration of a threaded tooth flank measuredon the workpiece before the milling of the tapping groove(s), as resultsfrom the command chain as per FIG. 13

The curve 93 in the figure comprises transitional functions which arebased on the inertia and the standard rigidity of the machine and thecontrol. The curve starts with a smooth course 94, and is abruptlyredirected at location 95, in synchronization with the jump command. Thelocation of maximum overshoot is location 96, which is followed by areturn swing 97. After this there is a small amplitude afterswing 98before the curve returns to a steady course 99.

FIG. 15 shows the lateral workpiece contour after the production of thetapping groove(s). The flanks of the tapping groove(s) are indicated bytwo dash-pointed lines 102, 103. These form the flanks 100, 101, of twothread teeth. The position of the tapping grooves is synchronized withthe contour of the threaded tooth flank in such a way that on the onehand the end 104 of the leading thread tooth is located in front of jumplocation 95, and on the other hand that an overstand with a relief angleis formed on cutting edge 105 at the following thread tooth. The smallbump 98 formed by the afterswing has an amplitude which is dependentboth on the mass and the control inertia of the system, as well as forexample on the cutting speed used. It is, however, of practically nosignificance for the general effectiveness of the primarily generatedprotruding cutting edge(s) and the relief angle.

The curve shown as an example in the drawing of two sequential threadedtooth flanks also includes a mutual swing of individual thread teeth intheir direction of propagation. The amount of this swing depends on thedesign specifications. The swing can be either minimized or completelyeradicated such that only a relict of the overshoot (96) remains in theform of cutting edge 105, or a part thereof, which extends beyond theend 104 of the leading thread tooth.

The method explained with the help of drawings 13 through 15 can beapplied in a corresponding fashion in for example flat threads on radialtooth heads (individual thread teeth) pointing outwards as well as onother threads on two or more surfaces of the threaded tooth profile.

A further application of the method according to the invention ispresented in FIGS. 16 and 17 based on an example. In this case this is aso-called circular wedge coupling which is used in general mechanicalengineering. FIG. 16 shows a coupling sleeve 106 with a center 107. Theinner wall has three circular wedge surface(s) 108, 109, 100, which abuton each other at jump(s) 111, 112 and 113. A journal 114 adapted to theinner profile of sleeve 106 is illustrated in FIG. 17. This journal hasthree outer circular wedge surface(s) 116, 117, 118 centred around thecentral axis 115 which cross over into one another at jump(s) 119, 120,121. The circular wedge surface(s) present on both sleeve 106 andjournal 114 are sections of spirals which end and begin abruptly at therespective abutment points. In order to produce these circular wedgesurface(s) using the method according to the invention it is inprinciple irrelevant whether these are sections from an archimedial, alogarithmic, hyperbolic or Fermatic spiral. One would, however,generally assume that a circular wedge surface is a section from alogarithmic spiral because this generates the most favorable materialloads during clamping because of the uniform angle of pitch.

When producing inner or outer circular wedge surfaces the significantaspect is that the curvature is mainly according to the specificationsand that the jumps waste as little as possible of the future contactarea. This task is achieved without any difficulties using the methodaccording to the invention including the jump system described in thepreceding. In order to cut for example the circular wedge sleeve 106 ona CNC lathe, a suitable blank is initially predrilled and if necessaryrough machined to achieve initial dimensions. The final machining usinga drill rod, for example with an index cutting tool is in principle suchthat during workpiece rotation the tool is moved radially outwards at aslow rate of advancement to the end of the circular wedge surface andthen is lifted from the circular wedge surface by a jump commanddirected inwards. This jump command in the program creates a tool trackcomprising a transitional element with an overshoot pointing towards thecenter 107 which is dimensioned in the programming such that the tool isa considerable distance from the start of the next circular wedgesurface. The following command blocks in the program are configured suchthat the next circular wedge surface is skipped and the tool isintroduced to the next but one circular wedge surface when its track hassettled. In the case of the example illustrated in FIG. 16, whichrequires a relative movement of the workpiece to the tool in aright-handed turn, looking in the direction of view, the machiningsequence of the three circular wedge surface(s) 108, 109, 110 would thenfor example be as follows, starting with circular wedge area 108:

-   108—machine from 112 to 111-   110—skip-   109—machine from 113 to 112-   108—skip-   110—machine from 111 to 113    -   109—skip    -   108—machine from 112 to 111    -   etc.

There are a number of freedoms with respect to the configuration of theNC program according to the invention. Thus for example the radialadvance can be programmed as pitch, with the choice of using asuperimposed modifying function, (e.g. using parameter E), or as fixedco-ordinates, in order to realize a specific form of surface curvature.As far as the axial tool movement is concerned there is the choice ofeither retaining the corresponding tool advance and hence using smalleradvance values or only employing advance either during the cutting ofthe individual circular wedge surface(s) or the cutting pauses duringskips.

The production of the journal required for the circular wedge surface(s)corresponds in principle the procedure described for the sleeve. Anappropriate tolerance of the dimensions should be borne in mind suchthat both parts fit together with the requisite gap. The jump surfacescreated by machining according to the invention only represent such asmall part of the circumference that between the fitted partners onlynegligible gaps are not used for the transfer of forces.

In fact the possibilities opened up by this method are virtuallyunlimited. They are generated by the application of CNC programs bylinking with the movement of a tool fixed to a carriage with therotation of the spindle and the inclusion or the combination of hobblevalues for the address parameters for diameter, length and pitch orspindle angle as well as the possibility of using a reciprocal-steptechnique or the described interleaved machining sequences. Thus it isnow possible to run machining tasks on CNC lathes extremely rationallywhich previously were very time consuming and in part had to be producedin poorer surface quality by milling.

The proposed artificial hip joint sockets with special threads andthread teeth of screw surfaces with neutral angles behind the cuttingedges as proposed for the application of the method is desirable becauseof the very low screw-in forces, extremely low risk of overtightening,excellent tactiliance and a for the most part gap-free transition to thebone bearing surface. A particularly advantageous model is such with apointed thread, tapping grooves with windup and thread teeth swungrelative to one another in the direction of the wind-up angle. This notonly makes handling of the screw socket considerably better duringimplantation but also substantially increases primary and secondaryfixation and hence virtually excludes the risk of premature loosening.

FIG. 18 is added for better understanding of FIG. 3. It shows a halfside sectional view A-A of the screw-in type artificial hip joint socket23 according to FIG. 3. The shell 28, having a bottom hole 31, isequipped with teeth 122, 123, 124 and 125. The teeth are realized in thetypical shape of a flat thread.

FIG. 19 shall serve for the same purpose as FIG. 18. It shows a halfside sectional view B-B of the screw-in type artificial hip joint socket34 according to FIG. 4. The shell, having a bottom hole 42 as before, isequipped with teeth 126, 127, 128 and 129, which now are realized as apointed thread.

FIG. 20 shows a turning lathe tool track program for a single jumpfunction as a schematic diagram. The program starts with a position A,then moves to a second position B, jumps to a position C, and moves onto a destination D. The general pitch of the turning lathe tool movementin this example then is present as the procession A to D.

FIG. 21 is used to show a simplified example of a double jump functionin form of a coordinate chain described by points E, F, G, H, J and K.Jump functions are existing from F to G and from H to J. The true tooltrack movement (not shown) will consist of transition elements dependingfrom the turning lathe turret response. The general pitch of the turninglathe tool movement then is present as the procession from E to K.

A group of simplified, quasi unwound portions, of parallel turning toolpaths are drawn out in FIG. 22. There is the choice of using twodifferent tools, the first one being used for path 1, 3 and 5, shown assolid lines, the second one for path 2, 4 and 6, shown as dashed lines,or a single tool for a first set of paths 1, 3 and 5, and a second setof paths 2, 4 and 6. This kind of programming is state-of-the-art andnot object of this patent application.

Compared to FIG. 22 described before, FIG. 23 shows an interlacing kindof programming according to the invention. There are two coordinatechains, a first one by 1E, 1F, 1G, 1H, 1J, 1K, drawn out as a solidline, and a second one by 2E, 2F, 2G, 2H, 2J and 2K, drawn out as adashed line. There is the choice of using two separate tools, one forthe first and another one for the second coordinate chain, or to use thesame tool for both tool paths. Within this example, the general pitch ofthe turning lathe tool track then is described as an interlacedprocession from 1E to 2J or from 1F to 2K, where jump functions or jumpsteps are present from 1F to 1G and from 1H to 1J, respectively from 2Fto 2G, and from 2H to 2J.

FIG. 24 shows two thread blades, also called thread wings or threadteeth 130 and 131, unwound from a threaded hip joint socket. The threadprofile of said thread blades is narrowing towards the profile head andtilted in direction to the socket pole, while its point is cut off.Flank angles and head width are identical for all thread blades, whilethe radial distance of said thread profile from the socket axis isconstant only for each single thread blade. Thus, each of said threadblades is surrounded by so called screw surfaces. The thread profile isshown as cut views 145, 146, 147 and 148 positioned at dot and dashlines 141, 142, 143 and 144. For the example shown a vertical rightflank angle of 0° was selected, and an inclined left flank angle of 26°(149, 150, 151, 152). Flattened sections of thread blades are drawn as134 and 135, and the left hand slope as 132, 133. Due to the threadpitch angle, thread blade bottoms are undergoing a height reduction from153 to 154, as well as from 155 to 156 relative to the socket mantle.The effect of this height reduction from the leading to the trailingportion of the individual thread blades is larger on the polar threadportion than on the equatorial portion of the thread. In the drawing,said height reduction is significantly enlarged for reasons of betterunderstanding. In order to create cutting surfaces 139, 140 and cuttingedges 157, 158 on said thread blades, tapping grooves—also calledflutes—are applied, indicated by dot and dash lines 136, 137, 138,having an angle with an identical winding direction compared to thethread pitch angle. The tapping groove angle was selected to be about45°. Thus, the cutting edges are highly “positive”, giving guarantee fora soft bone cutting behaviour and low cutting forces. For bringing saidbehaviour into effect, cutting edges of trailing thread blades have tobe exposed relatively to the trailing portion of leading thread blades.Therefore, according to the invention, said thread blades aresuperimposed by a type of “setting” (an angularly changed or swivelledextension), which in german language is correctly called “Schränkung”.

1. A lathing process for non-circular cutting on programmable lathingmachines whereby a workpiece is rotated in the chuck of a machinespindle and using a traversable tool at least partial specificnon-circular, discontinuous or abruptly changing contours are cut,wherein the turning takes place in jerks, in that the tool issynchronized to the spindle angle and the described contour comprisingrounded geometrical transition elements or compound contours isgenerated by a program comprising jump functions created by linkingcommand blocks with values for selected address parameters such asdiameter (X), length (Z), height (Y) and pitch (F) or angle (C), wherebyfor at least one of these address parameters the program block chainemploys a jerk, i.e. a sequence of address parameter values which buildinto a jump function.
 2. Process in accordance with claim 1 wherein thetool is moved relative to the workpiece on a track which can bedescribed as an alternating chain of smoothly flowing and jumpingcontour elements.
 3. Process in accordance with claim 1 wherein the toolis moved relative to the workpiece on a track which only corresponds ina single or in a number of isolated sections with the final contour andin which by way of at least one preceding and/or subsequent machiningcycles the superfluous, redundant, undesirable section or sectionnon-compliant with the final contour is remachined to the final contour.4. Process in accordance with claim 3 wherein the contour on theworkpiece is achieved by interleaving at least two machining sequenceswhereby e.g. in a first sequence the first contour element is machined,and then at least the next contour element is skipped and then asubsequent element is then machined and in at least one additionalsequence the skipped contour element or elements are machined and in sodoing the previously already machined contour elements are skipped. 5.Process in accordance with claim 3 wherein in each case respectivelymachined contour sections non-compliant with the final contour areremoved or remilled.
 6. Process in accordance with claim 1 wherein adriven tool is used in the machining of the contour which has a linearmovement of direction in at least one axis.
 7. Process in accordancewith claim 1 wherein the generation of the contour is achievedexclusively by the movement of the compound rest.
 8. Process inaccordance with claim 1 wherein a thread program is used.
 9. Process inaccordance with claim 1 wherein for at least two different addressparameters one jerk sequence of address parameter values is used in theprogram block chain.
 10. Process in accordance with claim 1 wherein forat least one of the address parameters the increments formed between theaddress parameter values of the program block chain are programmed asjerk sequences.
 11. Process in accordance with claim 1 wherein thediscontinuous contour is generated by the programming of areciprocal-step process in that the tool is traversed with a sequence offorward and backward movements whereby one of the movements is largerthan the other.
 12. Process according to claim 1 wherein the programblock chain describes a rotationally symmetric contour with asuperimposed non-monotonous periodic sequence of increments.
 13. Processin accordance with claim 1 for the cutting of discontinuous contourelements which protrude from an angled or curved shell surface wherebythe side of the tool predominantly machines the flank of thediscontinuous contour element and the tip of the tool predominantlymachines the surface shell, wherein the tip of the tool is guided on atrack which for the most part runs tangentially to the surface shell andin which the side of the tool is caused by a programmed modification ofthe tangential traverse speed and/or traverse direction to generate theflank of the discontinuous contour element.
 14. Process in accordancewith claim 1 wherein the overshoot behavior of the mechanical and/orelectronic systems of the lathe or the linear driven tool resulting forthe jump commend in the program is directly used for the generation ofdiscontinuous, non-round or abruptly changing contours of crookedcontours.
 15. Process in accordance with claim 14 wherein the overshootbehavior of the system is used for the direct creation of cutting edgeswith relief angles on thread segments or blades.
 16. Process inaccordance with claim 15 wherein the cutting edges are generated by atleast partial milling of cutting grooves in the area of a section of thethread blade resulting from the overshoot response of the system andrepresenting the relief angle relict of the overshoot.
 17. Applicationof the process in accordance with claim 1 for the cutting production ofspecial threads on screw-in bodies, e.g. for yielding material such asbone screws, lag screws, vertebral fusion bodies, screws for fixateurexterne, screw-in posts for dental implants and screw-in artificial hipjoint sockets in particular for the creation of neutral or any pinchingor relief angles on the thread blades.
 18. Application of the process inaccordance with claim 1 for the cutting production screw-in artificialhip joint sockets with any outer contour of the shell surface forexample spherical, paraspherical, conical, conical-spherical, parabolic,etc. and a thread on the shell surface which has a profile of the threadteeth with any tooth position, e.g. angled neutrally or towards the poleof the socket and any pitch, e.g. constant or variable pitch, withindividual thread blades separated from one another by cutting grooveswhich create any desired relief angle on at least one of the threadtooth surfaces.
 19. Application of the process in accordance with claim1 for the cutting production of screw-in artificial hip joint socketswith any outer contour of the shell surface for example spherical,paraspherical, conical, conical-spherical, parabolic, etc. and a threadon the shell surface which has a profile of the thread teeth with anytooth position, e.g. angled neutrally or towards the pole of the socketand any pitch, e.g. constant or variable pitch, with individual threadblades separated from one another by cutting grooves which createso-called screw surfaces on at least one of the thread tooth surfaces.20. Application of the process in accordance with claim 1 for thecutting production of screw-in bodies to create mutual swinging of thethread blades.
 21. Process in accordance with claim 1 wherein thenon-round or discontinuous contour comprises a closed surface withrepeating contour elements.
 22. Application of the process in accordancewith claim 21 for the production of circular wedge profiles or circularwedge couplings.